On the first 4 quizzes of his chemistry class, Omar got an average score of 78. What does he need on the next quiz to have an overall average of 82?
Solution: Let his score on the next quiz be $x$ The sum of all of his scores is then $4 \cdot 78 + x$ The same sum must also be equal to $5 \cdot 82$ Solve: $x = 5 \cdot 82 - 4 \cdot 78 = 98$.